1. Field of the Invention
Embodiments of the invention relate to identifying particles, and in particular to identifying particles in a liquid using illumination incident at an angle.
2. Background of the Invention
A major concern for, e.g., water utilities is the detection and control of pathogenic microorganisms, both known and emerging, in potable water treatment and distribution. There are not only a number of chlorine resistant pathogens such as Cryptosporidium that can contaminate drinking water systems, but also potentially harmful microorganisms that can be introduced, either accidentally or intentionally, and propagate under suitable environmental conditions. Due to the length of time for standard laboratory methods to yield results, typically 24-72 hours, there has not been a reliable system to detect microbial contaminants in real-time and on line to provide a warning of pathogen contamination events. Because of these expanding challenges, there has been an accelerated development of rapid tests and real-time methods to address the pressing needs of the water treatment community.
Conventional microbiological methods can be used to detect some harmful microorganisms; however, such methods provide limit results. Analytical methods in microbiology were developed over 120 years ago and are very similar today. These methods incorporate the following steps: sampling, culturing and isolating the microbes in a suitable growth media by incubation, identifying the organisms through microscopic examination or stains, and quantifying the organisms. Cryptosporidium and Giardia form oocysts or cysts and cannot be cultured in conventional ways. To detect these protozoan pathogens, an amount of water containing suspected pathogens, typically 10 liters, is sent through a special filter to collect and concentrate the organisms. Then the filter is eluded and the organisms further processed, such as staining the organisms and sending the concentrated solution through flow cytometry for example. These procedures, which can be found in Standard Methods or ASME, require ascetic technique in sampling and handling, skilled technicians to perform the analysis, and a number of reagents, materials, and instruments to obtain results. Practically, such methods have, therefore, proved to be time consuming, costly, and of little effectiveness for many current environmental field applications.
In order to reduce the amount of time to access microbiological results, a number of methods have been developed, mostly in the field of medicine. These faster tests have been improved and adapted to the environmental field and are generally categorized as 1) accelerated/automated tests 2) rapid tests and 3) contamination warning systems (CWS).
Accelerated tests are by grab sample and results can be obtained in 4 hours to 18 hours. Accelerated tests include immunoassays, ATP luminescence, and fluorescent antibody fixation. Rapid tests are also by grab sample and require manipulation of the sample to ‘tag’ the microbes with an identifiable marker or concentrate the microbe's genetic material (DNA) for subsequent identification. Results are normally available in 1-3 hours. These types of tests include Polymerase Chain Reaction (PCR) and Flow Cytometry.
Real time contamination warning systems are continuous warning devices that detect contaminants and provide an ‘event’ warning within minutes to prompt further investigation or action. CWS include laser based multi-angle light scattering (MALS) and multi-parameter chemical & particle instruments that detect water quality changes inferring potential biological contamination. Continuous, real time detection of pathogens in water surveillance was first discovered in the late 1960's and has progressed through a series of development steps until the first public field demonstration in 2002.
MALS is an acronym for “multi-angle light scattering” and is based on laser technology, photo-detection, and computer signal processing. When coherent light strikes a particle, a characteristic scattering pattern is emitted. The scattering pattern encompasses many features of the particle including the size, shape, internal structures (morphology), particle surface, and material composition (organic or inorganic). Each type of microorganism will scatter light giving off a unique pattern called a ‘bio-optical signature’. Photo-detectors collect the scattered light and capture the patterns which are then sent to an on-board computer. A microorganism's bio-optical signature is then compared against known pattern classifications in the detection library for results.
Presently, a detection system capable of meeting all of the ‘ideal detection system’ parameters, e.g., as cited by the American Water Works Association does not exist. Conventional MALS devices and methods often differ in the amount of time to obtain results, degree of specificity, sampling frequency, concentration sensitivity, operating complexity, and cost of ownership.